Supportive cellular elements for hepatic T cell differentiation: T cells expressing intermediate levels of the T cell receptor are cytotoxic against syngeneic hepatoma, and are lost after hepatocyte damage.

Extrathymic T cells exist in the liver and are often seen in close contact with Kupffer cells in the hepatic sinusoids. Since selective depletion of Kupffer cells has become possible by using liposome-encapsulated clodronate, it was investigated whether elimination of Kupffer cells influences the level of extrathymic T cells in the liver. Extrathymic T cells were identified as interleukin-2 receptor beta-chain (IL-2R beta) intermediate TCR (TCRint) cells by two-color staining for CD3 or T cell receptor (TCR) and IL-2R beta. The elimination of Kupffer cells did not significantly affect levels of TCRint cells up to 7 days after treatment. We then examined monocyte colony stimulating factor (M-CSF)-deficient op/op mice (low levels of Kupffer cells). Extrathymic T cells both in the liver and spleen of these mice were detected at a level comparable to that of control mice. Since extrathymic T cells in the liver are sometimes located in the parenchymal space, the relationship between extrathymic T cells and hepatocytes was then examined. Electron microscopy revealed that some hepatic T cells adhered directly to hepatocytes. When hepatocytes were damaged by a single injection of CCl4, hepatocyte death and subsequent hepatic fibrosis were induced. Beginning 3 days after injection, CD3int cells, but not other type of cells, decreased prominently. Purified CD3int cells, as well as whole lymphocytes in the liver, were cytotoxic against syngeneic hepatoma. In parallel with the above-mentioned hepatic damage, the cytotoxic activity of lymphocytes against such targets was impaired in the liver. These results suggest that extra-thymic generation of TCRint cells and their acquisition of cytotoxic function are relatively independent of Kupffer cells, but are dependent on hepatocytes.